The proposed research will focus on structural studies of guanine nucleotide dissociation inhibitor (GDI) and GDI-Rab protein complexes. Rab proteins are a ubiquitous family of small GTPases which regulate molecular events involved in vesicle targeting/fusion in both the exocytic and endocytic pathways. Members of the GDI family are escort proteins which control the recycling and prenylation of Rab. The specific objectives of this research proposal will reveal the molecular mechanism of the interaction between GDI and Rab to understand their physiological role in vesicular traffic. Specific Aim 1 will focus on the structural determination and refinement of GDI to 1.8 Angstroms resolution. This will provide the foundation for the structure determination of GDI-Rab complexes. Specific Aim 2 will focus on the structural analysis of the GDI-Rab complexes, using the molecular replacement method based on the results of Specific Aim l, or using new derivatives through the molecular isomorphous replacement (MI) method. Specific Aim 3 will focus on the crystallization of REP1, a related protein to GDI which is essential for critical post-translational modification of Rab proteins. Structural knowledge of the GDI-Rab complex will provide a foundation for understanding the biochemistry and molecular cytology of GDI function. Given the central role of GDI in membrane transport, these studies may provide a foundation for rational drug design to develop reagents in the future to augment these interactions in order to control GDI/REP1 function in the diseased state.